Catalytic Activity of Defect-Engineered Transition Me tal Dichalcogenides Mapped with Atomic-Scale Precision by Electrochemical Scanning Tunneling Microscopy.

Unraveling structure-activity relationships is a key objective of catalysis. Unfortunately, the intrinsic complexity and structural heterogeneity of materials stand in the way of this goal, mainly because the activity measurements are area-averaged and therefore contain information coming from different surface sites. This limitation can be surpassed by the analysis of the noise in the current of electrochemical scanning tunneling microscopy (EC-STM). Herein, we apply this strategy to investigate the catalytic activity toward the hydrogen evolution reaction of monolayer films of MoSe2. Thanks to atomically resolved potentiodynamic experiments, we can evaluate individually the catalytic activity of the MoSe2 basal plane, selenium vacancies, and different point defects produced by the intersections of metallic twin boundaries. The activity trend deduced by EC-STM is independently confirmed by density functional theory calculations, which also indicate that, on the metallic twin boundary crossings, the hydrogen adsorption energy is almost thermoneutral. The micro- and macroscopic measurements are combined to extract the turnover frequency of different sites, obtaining for the most active ones a value of 30 s-1 at -136 mV vs RHE.

Synergistic Effect of Sn and Fe in Fe-Nx Site Formation and Activity in Fe-N-C Catalyst for ORR.

Iron-nitrogen-carbon (Fe-N-C) materials emerged as one of the best non-platinum group material (non-PGM) alternatives to Pt/C catalysts for the electrochemical reduction of O2 in fuel cells. Co-doping with a secondary metal center is a possible choice to further enhance the activity toward oxygen reduction reaction (ORR). Here, classical Fe-N-C materials were co-doped with Sn as a secondary metal center. Sn-N-C according to the literature shows excellent activity, in particular in the fuel cell setup; here, the same catalyst shows a non-negligible activity in 0.5 M H2SO4 electrolyte but not as high as expected, meaning the different and uncertain nature of active sites. On the other hand, in mixed Fe, Sn-N-C catalysts, the presence of Sn improves the catalytic activity that is linked to a higher Fe-N4 site density, whereas the possible synergistic interaction of Fe-N4 and Sn-Nx found no confirmation. The presence of Fe-N4 and Sn-Nx was thoroughly determined by extended X-ray absorption fine structure and NO stripping technique; furthermore, besides the typical voltammetric technique, the catalytic activity of Fe-N-C catalyst was determined and also compared with that of the gas diffusion electrode (GDE), which allows a fast and reliable screening for possible implementation in a full cell. This paper therefore explores the effect of Sn on the formation, activity, and selectivity of Fe-N-C catalysts in both acid and alkaline media by tuning the Sn/Fe ratio in the synthetic procedure, with the ratio 1/2 showing the best activity, even higher than that of the iron-only containing sample (jk = 2.11 vs 1.83 A g-1). Pt-free materials are also tested for ORR in GDE setup in both performance and durability tests.

Electrochemical Investigation of Iron-Catalyzed Atom Transfer Radical Polymerization.

Use of iron-based catalysts in atom transfer radical polymerization (ATRP) is very interesting because of the abundance of the metal and its biocompatibility. Although the mechanism of action is not well understood yet, iron halide salts are usually used as catalysts, often in the presence of nitrogen or phosphorous ligands (L). In this study, electrochemically mediated ATRP (eATRP) of methyl methacrylate (MMA) catalyzed by FeCl3, both in the absence and presence of additional ligands, was investigated in dimethylformamide. The electrochemical behavior of FeCl3 and FeCl3/L was deeply investigated showing the speciation of Fe(III) and Fe(II) and the role played by added ligands. It is shown that amine ligands form stable iron complexes, whereas phosphines act as reducing agents. eATRP of MMA catalyzed by FeCl3 was investigated in different conditions. In particular, the effects of temperature, catalyst concentration, catalyst-to-initiator ratio, halide ion excess and added ligands were investigated. In general, polymerization was moderately fast but difficult to control. Surprisingly, the best results were obtained with FeCl3 without any other ligand. Electrogenerated Fe(II) effectively activates the dormant chains but deactivation of the propagating radicals by Fe(III) species is less efficient, resulting in dispersity > 1.5, unless a high concentration of FeCl3 is used.

H2O2 production at gas-diffusion cathodes made from agarose-derived carbons with different textural properties for acebutolol degradation in chloride media.

The excessive cost, unsustainability or complex production of new highly selective electrocatalysts for H2O2 production, especially noble-metal-based ones, is prohibitive in the water treatment sector. To solve this conundrum, biomass-derived carbons with adequate textural properties were synthesized via agarose double-step pyrolysis followed by steam activation. A longer steam treatment enhanced the graphitization and porosity, even surpassing commercial carbon black. Steam treatment for 20 min yielded the greatest surface area (1248 m2 g-1), enhanced the mesopore/micropore volume distribution and increased the activity (E1/2 = 0.609 V) and yield of H2O2 (40%) as determined by RRDE. The upgraded textural properties had very positive impact on the ability of the corresponding gas-diffusion electrodes (GDEs) to accumulate H2O2, reaching Faradaic current efficiencies of ~95% at 30 min. Acidic solutions of ß-blocker acebutolol were treated by photoelectro-Fenton (PEF) process in synthetic media with and without chloride. In urban wastewater, total drug disappearance was reached at 60 min with almost 50% mineralization after 360 min at only 10 mA cm-2. Up to 14 degradation products were identified in the Cl--containing medium.

Sulfur Doping versus Hierarchical Pore Structure: The Dominating Effect on the Fe-N-C Site Density, Activity, and Selectivity in Oxygen Reduction Reaction Electrocatalysis.

Nitrogen doping has been always regarded as one of the major factors responsible for the increased catalytic activity of Fe-N-C catalysts in the oxygen reduction reaction, and recently, sulfur has emerged as a co-doping element capable of increasing the catalytic activity even more because of electronic effects, which modify the d-band center of the Fe-N-C catalysts or because of its capability to increase the Fe-Nx site density (SD). Herein, we investigate in detail the effect of sulfur doping of carbon support on the Fe-Nx site formation and on the textural properties (micro- and mesopore surface area and volume) in the resulting Fe-N-C catalysts. The Fe-N-C catalysts were prepared from mesoporous carbon with tunable sulfur doping (0-16 wt %), which was achieved by the modulation of the relative amount of sucrose/dibenzothiophene precursors. The carbon with the highest sulfur content was also activated through steam treatment at 800 °C for different durations, which allowed us to modulate the carbon pore volume and surface area (1296-1726 m2 g-1). The resulting catalysts were tested in O2-saturated 0.5 M H2SO4 electrolyte, and the site density (SD) was determined using the NO-stripping technique. Here, we demonstrate that sulfur doping has a porogenic effect increasing the microporosity of the carbon support, and it also facilitates the nitrogen fixation on the carbon support as well as the formation of Fe-Nx sites. It was found that the Fe-N-C catalytic activity [E1/2 ranges between 0.609 and 0.731 V vs reversible hydrogen electrode (RHE)] does not directly depend on sulfur content, but rather on the microporous surface and therefore any electronic effect appears not to be determinant as confirmed by X-ray photoemission spectroscopy (XPS). The graph reporting Fe-Nx SD versus sulfur content assumes a volcano-like shape, where the maximum value is obtained for a sulfur/iron ratio close to 18, i.e., a too high or too low sulfur doping has a detrimental effect on Fe-Nx formation. However, it was highlighted that the increase of Fe-Nx SD is a necessary but not sufficient condition for increasing the catalytic activity of the material, unless the textural properties are also optimized, i.e., there must be an optimized hierarchical porosity that facilitates the mass transport to the active sites.

PEO-b-PS Block Copolymer Templated Mesoporous Carbons: A Comparative Study of Nitrogen and Sulfur Doping in the Oxygen Reduction Reaction to Hydrogen Peroxide.

Carbon materials slightly doped with heteroatoms such as nitrogen (N-RFC) or sulfur (S-RFC) are investigated as active catalysts for the electrochemical bielectronic oxygen reduction reaction (ORR) to H2 O2 . Mesoporous carbons with wide, accessible pores were prepared by pyrolysis of a resorcinol-formaldehyde resin using a PEO-b-PS block copolymer as a sacrificial templating agent and the nitrogen and sulfur doping were accomplished in a second thermal treatment employing 1,10-phenanthroline and dibenzothiophene as nitrogen and sulfur precursors, respectively. The synthetic strategy allowed to obtain carbon materials with very high surface area and mesopore volume without any further physicochemical post treatment. Voltammetric rotating ring-disk measurements in combination with potentiostatic and galvanostatic bulk electrolysis measurements in 0.5 m H2 SO4 demonstrated a pronounced effect of heteroatom doping and mesopores volume on the catalytic activity and selectivity for H2 O2 . N-RFC electrode was employed as electrode material in a 45 h electrolysis showing a constant H2 O2 production of 298 mmol g-1 h-1 (millimoles of H2 O2 divided by mass of catalyst and electrolysis time), with a faradic efficiency (FE) up to 61 % and without any clear evidence of degradation. The undoped carbon RFC showed a lower production rate (218 mmol g-1 h-1 ) but a higher FE of 76 %, while the performances drastically dropped when S-RFC (production rate 11 mmol g-1 h-1 and FE=39 %) was used.

Mesoporous Carbon with Different Density of Thiophenic-Like Functional Groups and Their Effect on Oxygen Reduction.

The metal-support interactions between sulfur-doped carbon supports (SMCs) and Pt nanoparticles (NPs) were investigated, aiming at verifying how sulfur functional groups can improve the electrocatalytic performance of Pt NPs towards the oxygen reduction reaction (ORR). SMCs were synthetized, tailoring the density of sulfur functional groups, and Pt NPs were deposited by thermal reduction of Pt(acac)2 . The extent of the metal-support interaction was proved by X-ray photoelectron spectroscopy (XPS) analysis, which revealed a strong electronic interaction, proportional to the density of sulfur defects, whereas XRD spectra provided evidence of higher strain in Pt NPs loaded on SMC. DFT simulations confirmed that the metal-support interaction was strongest in the presence of a high density of sulfur defects. The combination of microstrain and electronic effects resulted in a high catalytic activity of supported Pt NPs towards ORR, with linear correlations of the half-wave potential E1/2 or the kinetic current jk with the sulfur content in the support. Furthermore, a mass activity value (550 A g-1 ) well above the United States Department of Energy target of 440 A g-1 at 0.9 V (vs. reversible hydrogen electrode, RHE), was determined.

A Review on Surface-Enhanced Raman Scattering.

Surface-enhanced Raman scattering (SERS) has become a powerful tool in chemical, material and life sciences, owing to its intrinsic features (i.e., fingerprint recognition capabilities and high sensitivity) and to the technological advancements that have lowered the cost of the instruments and improved their sensitivity and user-friendliness. We provide an overview of the most significant aspects of SERS. First, the phenomena at the basis of the SERS amplification are described. Then, the measurement of the enhancement and the key factors that determine it (the materials, the hot spots, and the analyte-surface distance) are discussed. A section is dedicated to the analysis of the relevant factors for the choice of the excitation wavelength in a SERS experiment. Several types of substrates and fabrication methods are illustrated, along with some examples of the coupling of SERS with separation and capturing techniques. Finally, a representative selection of applications in the biomedical field, with direct and indirect protocols, is provided. We intentionally avoided using a highly technical language and, whenever possible, intuitive explanations of the involved phenomena are provided, in order to make this review suitable to scientists with different degrees of specialization in this field.

Clean rhodium nanoparticles prepared by laser ablation in liquid for high performance electrocatalysis of the hydrogen evolution reaction.

Rhodium nanoparticles (NPs) were prepared by a one-step, green and facile procedure consisting in laser ablation of a bulk Rh target immersed in pure water (W-Rh-NPs) or ethanol (E-Rh-NPs). When embedded in mesoporous carbon based inks, both W-Rh-NPs and E-Rh-NPs show excellent activity towards the hydrogen evolution reaction in acidic media, operating close to the thermodynamic potential with 85-97% faradaic yields and low Tafel slopes of 50-54 mV per decade in the low overpotential region (η < 20 mV). A superior activity of W-Rh-NPs with respect to E-Rh-NPs is ascribed to the absence of surface carbon reducible species derived from the synthesis in organic solvent, and thus confirms the importance of the use of water as the preferred medium for laser synthesis of clean nanocrystals in liquid environment. These results provide an important contribution to the impelling need for the preparation of nano-catalysts based on energy critical materials by clean, sustainable and low cost routes.

Physico-Chemical, Electrochemical and Structural Insights Into Poly(3,4-ethylenedioxythiophene) Grafted from Molecularly Engineered Multi-Walled Carbon Nanotube Surfaces.

Composites of multi-walled carbon nanotubes (MWCNTs) and poly(3,4-ethylenedioxythiophene) (PEDOT) are attracting the attention of material scientists since more than a decade as potential next-generation optoelectronic materials for their peculiar features, arising from the combination of the intrinsic electrical, thermal and morphological properties of the two components. They are indeed a promising platform for the development of low-cost, portable and environmentally friendly electronic devices such as supercapacitors, sensors and actuators. Here a novel synthetic strategy for their preparation is envisaged, exploiting the possibility to covalently functionalize the external surface of MWCNTs with tailored molecular units, starting from which the growth of the conjugated polymer can be induced oxidatively. The approach demonstrates its value in being able to effectively promote the formation of PEDOT chains in direct contact with the surface of MWCNTs, differently from what results when the monomer is polymerized in the presence of the pristine carbon nanomaterial. In addition, significant differences are found in the physico-chemical properties and electrochemical behavior when MWCNT-PEDOT covalent composites are studied in comparison to a non-covalent analogue, here illustrated in detail. These evidences constitute a starting point for the future development of novel more finely tuned functional materials based on MWCNT-PEDOT composites, featuring the required optoelectronic properties to precisely target the desired application.

Reductive cleavage of carbon-chlorine bonds at catalytic and non-catalytic electrodes in 1-butyl-3-methylimidazolium tetrafluoroborate.

Dissociative electron transfer (DET) to a series of organic chlorides at glassy carbon (GC), silver and copper electrodes has been studied in 1-butyl-3-methylimidazolium tetrafluoroborate. The overall results of this study show that the ionic liquid behaves like molecular solvents such as acetonitrile and dimethylfomamide. It is found that aromatic chlorides follow a stepwise mechanism, whereas concerted electron transfer/bond cleavage is the preferred reaction mechanism for alkyl and benzyl chlorides. Ag and Cu show catalytic effects only when the DET follows a concerted mechanism, but Ag proves to be a much better electrocatalyst than Cu. A series of substituted benzyl chlorides (Z-C6H4CH2Cl, Z = H, 3-OCH3, 3-F, 4-Cl, and 3-CF3) show interesting results providing some insight into the reaction dynamics. The process occurs by a concerted mechanism and, albeit a constant standard potential for the whole series, Ep on GC and Cu, which does not show catalytic activity, is significantly affected by the substituents. In contrast, Ag shows good catalytic activity and, as expected, Ep does not change with the substituent. This difference in behavior may be rationalized by considering ion-dipole interactions between R˙ and Cl(-) as opposed to adsorption of the fragments on the electrode surface.

Metal-support interaction in platinum and palladium nanoparticles loaded on nitrogen-doped mesoporous carbon for oxygen reduction reaction.

Mesoporous carbons are highly porous materials, which show large surface area, chemical inertness and electrochemical performances superior to traditional carbon material. In this study, we report the preparation of nitrogen-doped and undoped mesoporous carbons by an optimized hard template procedure employing silica as template, sucrose and ammonia as carbon and nitrogen source, respectively. Surface area measurements assert a value of 900 and 600 m(2) g(-1) for the best doped and undoped samples, respectively. Such supports were then thoroughly characterized by surface science and electron microscopy tools. Afterward, they were decorated with Pt and Pd nanoparticles, and it was found that the presence of nitrogen defects plays a significant role in improving the metal particles dimension and dispersion. In fact, when doped supports are used, the resulting metal nanoparticles are smaller (2-4 nm) and less prone to aggregation. Photoemission measurements give evidence of a binding energy shift, which is consistent with the presence of an electronic interaction between nitrogen atoms and the metal nanoparticles, especially in the case of Pd. The catalytic properties of electrodes decorated with such catalyst/support systems were investigated by linear sweep voltammetry and by rotating disk electrode measurements, revealing excellent stability and good activity toward oxygen reduction reaction (ORR). In particular, although Pd nanoparticles always result in lower activity than Pt ones, both Pt and Pd electrodes based on the N-doped supports show an increased activity toward ORR with respect to the undoped ones. At the same mass loading, the Tafel slope and the stability test of the Pt@N-doped electrocatalysts indicate superior performances to that of a commercial Pt@C catalysts (30 wt % Pt on Vulcan XC-72, Johnson Matthey).

Palladium nanoparticles supported on nitrogen-doped HOPG: a surface science and electrochemical study.

We have investigated by photoemission spectroscopy and scanning tunnelling microscopy what are the chemical and structural changes induced by nitrogen ion implantation (500 eV) on highly oriented pyrolytic graphite and how the defects induced by this process modify the growth and thermal stability of palladium nanoparticles, deposited in situ by physical vapour deposition. Since nitrogen derived defects are mostly buried below the surface, they are not accessible for a chemical interaction with metal nanoparticles; however, the amorphization induced by the ion beam in the outermost layers of the substrate beneficially affects the metal morphology, limiting the size of the nanoparticles and improving their thermal stability. The supported nanoparticles have been tested towards the oxygen reduction reaction indicating that the electrochemical activity does not depend significantly on the ion implantation, but mostly on the amount of palladium.

Advanced oxidation processes coupled with electrocoagulation for the exhaustive abatement of Cr-EDTA.

Using Cr-EDTA as a model system, a two-step method has been investigated for the abatement of persistent chromium complexes in water. The treatment consists of an oxidative decomposition of the organic ligands by means of ozonization or electrochemical oxidation at a boron doped diamond (BDD) electrode, followed by removal of the metal via electrochemical coagulation. In the designed synthetic waste, EDTA has been used both as a chelating agent and as a mimic of the organic content of a typical wastewater provided by a purification leather plant. A crucial point evaluated is the influence of the oxidative pretreatment on the chemical modification of the synthetic waste and hence on the electrocoagulation efficacy. Because of the great stability of Cr complexes, such as Cr-EDTA, the classical coagulation methods, based on ligand exchange between Cr(III) and Fe(II) or Fe(III), are ineffective toward Cr abatement in the presence of organic substances. On the contrary, when advanced oxidation processes (AOPs), such as ozonization or electrooxidation at a BDD anode are applied in series with electrocoagulation (EC), complete abatement of the recalcitrant Cr fraction can be achieved. ECs have been carried out by using Fe sacrificial anodes, with alternating polarization and complete Cr abatement (over 99%) has been obtained with modest charge consumption. It has been found that Cr(III) is first oxidized to Cr(VI) in the AOP preceding EC. Then, during EC, Cr(VI) is mainly reduced back to Cr(III) by electrogenerated Fe(II). Thus, Cr is mainly eliminated as Cr(III). However, a small fraction of Cr(VI) goes with the precipitate as confirmed by XPS analysis of the sludge.

Exhaustive depletion of recalcitrant chromium fractions in a real wastewater.

This paper reports investigations on electrochemical removal of Cr(III) from a real wastewater sample from a 4x10(4)m(3)d(-1) purification plant located in the leather district of Arzignano (Italy). A crucial point evaluated is the relation between Cr(III) abatement and the reduction of total organic carbon and chemical oxygen demand. Here we present the results of different abatement approaches including advanced oxidation processes (AOPs) and electrocoagulation applied separately or in series. Neither of these methods show satisfactory results when applied alone. In particular, AOPs are completely inefficient owing to the formation of Cr(VI), which is more soluble than Cr(III). Almost total depletion (99.7%) of the recalcitrant fraction of Cr(III) is successfully achieved by combining ozonization and electrocoagulation processes. The results are consistent with the abatement of Cr in its hexavalent form by fast precipitation as NH(4)Fe(CrO(4))(2).

Dissociative electron transfer to organic chlorides: electrocatalysis at metal cathodes.

The reductive cleavage of a series of organic chlorides, including chloroaromatics, benzyl chlorides, activated chloroalkanes and polychloromethanes, was investigated at Ag, Cu, Pd and glassy carbon (GC) electrodes in CH(3)CN + 0.1 M (C(2)H(5))(4)NClO(4). The silver cathode was either a 2-mm diameter disc, fabricated from Ag wire, or nanoclusters of average diameter d = 304 nm, prepared by electrodeposition on GC. Ag, Cu and Pd electrodes have shown remarkable electrocatalytic properties for the reduction of several compounds. The peak potentials recorded at these electrodes, for example, at upsilon = 0.1 V s(-1) are positively shifted by 0.3-0.8 V with respect to the reduction potentials measured at a non catalytic electrode such as GC. Electrocatalysis is strictly related to the concerted nature of the dissociative electron transfer to the carbon-chlorine bond. No catalysis is observed when the dissociative electron transfer to RCl occurs according to a stepwise mechanism involving the intermediate formation of a radical anion. The catalytic surfaces affect the reaction scheme, offering a more favourable route possibly through the formation of strongly adsorbed activated complexes.

Metal-metal electronic coupling in syn and anti stereoisomers of mixed-valent (FeCp)2-, (RhL2)2-, and (FeCp)(RhL2)-as-indacenediide ions.

The extent of metal-metal electronic coupling was quantified for a series of syn and anti stereoisomers of (FeCp)(2)-, (RhL(2))(2)- and (FeCp)(RhL(2))- (L(2)=1,5-cyclooctadiene (cod), L=CO) as-indacenediide mixed-valent ions by spectroelectrochemical and DFT studies. The effect of the syn/anti orientation of the metal units with respect to the planar aromatic ligand indicates that electron transfer occurs through the bridge rather than through space. The nature of the metal was found to be crucial: while homobimetallic diiron species are localised valence-trapped ions (Class II), the dirhodium analogues are almost delocalised mixed-valent ions (borderline and Class III). Finally, despite their redox asymmetry, even in the heterobimetallic iron-rhodium as-indacenediide complexes, strong metal-metal coupling is present. In fact, oxidation of the iron centre is accompanied by electron transfer from rhodium to iron and formation of a reactive 17-electron rhodium site. syn and anti Fe-Rh as-indacenediide complexes are rare examples of heterobimetallic systems which can be classified as borderline Class II/Class III species.

Tuning the electronic communication in heterobimetallic mixed-valence ions of (1-ferrocenyl)- and (2-ferrocenyl)indenyl rhodium isomers.

A series of heterobimetallic complexes of general structure [RhL(2){eta(5)-(2-ferrocenyl)indenyl}] (L(2)=cod, nbd, L=CO; cod=cyclooctadiene; nbd=norbornadiene) has been synthesised with the aim of tuning the metal-metal interaction in their mixed-valence ions generated both by chemical and electrochemical oxidation, and the results are compared with those obtained for [RhL(2){eta(5)-(1-ferrocenyl)indenyl}] isomers. Crystallographic studies and DFT calculations provide a detailed description of the structural and electronic features of these complexes evidencing a significant difference in the extent of planarity of the flexible bridging ligand between the 1- and 2-ferrocenyl isomers. Independent experimental probes, in particular the potential splitting in the cyclic voltammograms and the IT bands in the near-IR spectra, are rationalised in the framework of Marcus-Hush theory and at quantum chemistry level by DFT and TD-DFT methods. These methods allow us to establish a trend based on the magnitude of iron-rhodium electronic coupling H(ab) ranging from valence trapped to almost delocalised ions. The quasi planar bridge and the olefin ancillary ligands make [Rh(nbd){eta(5)-(2-ferrocenyl)indenyl}](+) and [Rh(cod){eta(5)-(2-ferrocenyl)indenyl}](+) rare examples of heterobimetallic systems which can be classified as borderline Class II/Class III species.